In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Multi-antenna techniques can significantly increase data rates and/or reliability of a wireless communication system. Performance may be improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In a downlink from a base station transmitting from an antenna array over a MIMO channel to a wireless terminal, spatial multiplexing (according to the spatial multiplexing mode) may allow the simultaneous transmission of multiple symbol streams over the same frequency from different antennas of the base station antenna array. Stated in other words, multiple symbol streams may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide an increased data rate. In a downlink from the same base station transmitting from the same antenna array to the same wireless terminal, diversity (according to the diversity mode) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the base station antenna array. Stated in other words, the same symbol stream may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the wireless terminal due to diversity gain.
The performance of a wireless communication system can thus be improved using multiple antennas at the base station and wireless terminal to provide spatial multiplexing in more favorable channel conditions and to provide diversity gain in less favorable channel conditions. In general, the wireless terminal decides whether to receive in a MIMO diversity mode or a MIMO multiplexed mode, and the wireless terminal reports this decision using a Rank Indicator (RI). For example, a Rank. Indicator of 1 may specify a MIMO diversity mode, and a Rank Indicator of 2 may specify a MIMO multiplexed mode. Moreover, because MIMO downlink channel characteristics vary as a function of many factors including location of the wireless terminal relative to the base station, direction/speed of movement of the wireless terminal, natural and manmade terrain, etc., and because MIMO downlink characteristics and demand for downlink channel capacity may change during a communication, the Rank Indicator (RI) calculations and channel information reporting may be performed by the wireless terminal over the entire band (for example, in 3GPP-LTE, Wi-Max, etc.) at given periods as configured by the base station.
Rank Indicator (RI) choice is generally a function of channel characteristics, correlation between transmit and receive antennas, etc. A conventional approach to choosing a Rank Indicator (RI) is to find Signal to Interference-plus-Noise Ratios (SINRs) for different transmission configurations, and to choose a Rank Indicator for a configuration providing a highest spectral efficiency. In open loop MIMO systems, for example, a wireless terminal may compute a SINR for each possible value of RI and find spectral efficiencies using an exhaustive search of all possible configurations. The wireless terminal may then choose the Rank Indicator for the configuration providing the highest spectral efficiency. If spectral efficiencies of two Rank Indicators are equal, the wireless terminal may then choose the Rank Indicator that maximizes the SINR. The wireless terminal may then repon the selected Rank Indicator (RI). Modulation and Coding Scheme (MSC), and/or Channel Quality Information (CQI) to the base station in a Channel State Information (CSI) report. Similarly burdensome operations may be required for Rank Indicator (RI) and/or Precoder Matrix Indicator (PMI) calculations in closed loop MIMO systems, particularly when the codebook is large.
The requirement for the wireless terminal to periodically determine spectral efficiencies for all possible downlink configurations may place a burden on wireless terminal processing capacity and/or may reduce wireless terminal battery life. Accordingly, there continues to exist a need in the art for methods that reduce a processing capacity required for Channel State Information (CSI) reporting.